This invention relates to fluid volumetric flow sensors that employ rotating elements, and particularly to improvements in the rotor support.
Numerous devices have been proposed for measuring volumetric flow of fluids. Many of these use a rotating element supported on a bearing or bearings. Bearings in these meters have historically been expensive to make, fragile (particularly with respect to overspeed damage), and prone to both wear and corrosion.
McNabb, in U.S. Pat. No. 3,447,373, provided seminal teaching of a "bearingless", or liquid bearing, flowmeter in which the moving fluid not only rotated a wheel element in a chamber, but also held the rotor element out of contact with the walls of the chamber. The disclosure of McNabb is herein incorporated by reference.
As a matter of practice, dimensional tolerances on the order of 2.5 .mu.m (0.0001 inch) are required for a meter of the McNabb design to function satisfactorily. McNabb meters are plagued with instabilities traceable to small dimensional variations. Specifically identified instabilities include:
axial wobble, in which the rotor oscillates about an axis perpendicular to its normal axis of rotation; PA1 planar wobble, in which the rotor is translated back and forth perpendicular to its normal axis of rotation; PA1 step frequency shifts, in which non-linear changes are observed in the rate of rotor rotation at constant flow speed (these are due to the rotor changing between various possible vibratory modes); and PA1 spiking, in which the rate of rotor rotation at constant flow rate drops abruptly before returning to its previous rate (these are due to the rotor hitting the wall of the rotor chamber).
Instabilities have persisted in McNabb meters for many years, in spite of repeated attempts (e.g., those of Hoppe and of Bullock et al cited hereinafter) to eliminate them by improved mechanical design. To date these have proven unsuccessful, and compensation for the instabilities has been provided via computer software in the signal processing electronics.
The tight dimensional tolerances inherent in the McNabb design require in-line filters to remove particulate contamination that otherwise clogs the instrument and require its being taken out of service for cleaning. Additionally, contaminant buildup on the surfaces of moving parts in the McNabb meter lead to upward shifts of the operating frequency.
The rotor element in a McNabb meter must be small and lightweight and have a density closely approximating that of the density of the fluid being measured. These requirements lead to the use of optical means of sensing the rotational velocity of the wheel. Since the McNabb meter is generally made with an opaque plastic or metal body, optical sensing generally requires the use of fiber optics, which impose additional problems of high assembly labor cost, leakage around the fibers, and unpredictable variations in output when contaminants build up on the internal surfaces of the instrument.
Moreover, the start-up characteristics of the McNabb transducer are unpredictable, since the rotor does not have a predetermined setting for a no-flow condition. At rest, varying areas on the rotor of McNabb's apparatus may contact any of several internal surfaces of the instrument, depending on the relative specific gravity of the rotor and fluid and on the mounting attitude of the transducer. Thus, start/stop measurements made with a McNabb flowmeter are unreliable. Start/stop (or metered volume) measurements constitute an important fraction of flow measurement applications and include, for example, a volumetric meter in a soft-drink vending machine that dispenses a first predetermined volume of carbonated water and a second predetermined volume of flavored syrup into a cup during each vend.
Hoppe, in U.S. Pat. No. 3,927,568, taught a number of rotor geometries to improve the linearity of a McNabb flow transducer.
Hoppe, in U.S. Pat. No. 4,015,474, subsequently taught an improvement to the McNabb design that improved rotor stability in a low flow rate regime. At commonly encountered volumetric flow rates Hoppe's device was subject to both axial and planar wobble.
Bullock et al, in U.S. Pat. No. 4,833,925, taught the use of a asymmetrical and unbalanced rotor element to reduce the severity of random frequency step shifts in transducers of the basic McNabb design. Although the unbalanced rotor element aided in reducing step shifts, it worsened wobble problems and degraded signal quality.